Western Boundary Currents Forcing on the Southwestern Atlantic Shelf

PALMA, E. D.; MATANO, R. P.; PIOLA, A. R.

Foz do Iguacu

Congreso; 2010 Meeting of the Americas; 2010

American Geophysical Union

The Southwestern Atlantic Shelf is the largest continental shelf of the southern hemisphere, andone of the most biologically productive areas of the world ocean. Along its eastern margin, theneighboring western boundary currents, Brazil and Malvinas Currents, flow in opposition andcollide near 38°S. In this study we aim to improve our knowledge of the circulation patterns overthis vast region through the analysis of long-term numerical simulations that includes all themajor forcing of the region: winds, tides, freshwater discharges, and the western boundarycurrents. The numerical experiments indicate that on the southern Patagonian Shelf thecirculation consists of a general northeastward flow controlled by low salinity discharges fromMagellan Strait, tidal mixing and wind forcing, but north of 50°S the influence of the MalvinasCurrent controls the magnitude of the along-shelf transport. In the offshore region the mostconspicuous aspect of the salinity field is the shelf break front, which is characterized by strongupwelling velocities whose dynamical origin is linked with the lateral spreading of the MalvinasCurrent onto the shelf. The region from 40°S to 33°S presents intrusions of cold subantarcticwaters and the northward expansion of mixtures of the Río de la Plata waters in late fall and aslower retraction of the plume during spring-summer. Offshore, near the collision point,hydrographic observations and model results show that a mixture of low salinity waters isexported to the deep-ocean. The analysis also indicates that the influence of the MalvinasCurrent is felt well beyond its retroflection point in the form of a northward middle-shelf currentthat during summer flows against the northwesterly prevailing winds. This current is generatedby the upstream spreading (in the arrested topographic wave sense) of pressure gradientsset-up farther south by the Malvinas Current. From 35°S to 20°S the interaction between thepoleward flow of the Brazil Current and the bottom topography greatly influences the shelfcirculation, particularly in the bottom boundary layer. As the boundary current flows along thecontinental slope, changes in the coastline orientation and along-shore bottom topographymodify the along-shore pressure gradient which through geostrophy leads to inshore bottomflow and hence yearlong shelf-break upwelling. The subsequent entrainment of slope watersonto the shelf is mainly controlled by wind forcing.